Thermal spraying is a generic term for a group of industrial processes involving the feeding of a desirable or heat-fusible material into a heating zone to be melted, or at least heat-softened, and then propelled from the heating zone in a finely divided form, generally, for depositing metallic or nonmetallic coatings on a substrate. Thermal spraying was mostly used during the initial stages of its commercial development for spraying metals to repair or build up worn, damaged, or improperly machined parts. Recently, however, a much wider group of materials, including refractory alloys, ceramics, cermets, carbides and other compounds are used to impart wear, corrosion, or oxidation resistance to the base material. These processes, sometimes still collectively called metalizing, broadly include flame spraying, electric-arc spraying and plasma-arc spraying.
These three basic types differ, primarily, in the type of equipment used for the heating zone. Flame spraying utilizes combustible fuel gas (such as acetylene, propane, natural gas, or sometimes hydrogen) which reacts with oxygen or air. Electric-arc and plasma-arc utilize, naturally, electrical energy to produce the heating zone. Additionally, a blast gas may be provided in order to aid in accelerating the heated particles and propelling them from the heating zone toward the surface to be coated and/or to cool the workpiece and the coating being formed thereon.
The detailed characteristics, as well as advantages and disadvantages, of these three basic types of thermal spraying processes are discussed in Volume 5 of the Metals Handbook Ninth Edition (pp. 361-368) which is incorporated herein by reference.
The coating material can initially be wire or rod stock, or powdered material. If in the form of wire or rod, it is fed into the heating zone where it is melted. The molten stock is then stripped from the end of the wire or rod and atomized by a high velocity stream of compressed air or other gas which propels the material onto a prepared substrate or workpiece. If in powdered form, the material is usually metered by a powder feeder or hopper into a compressed air or gas stream which suspends and delivers it to the heating zone. The characteristics of suitable flame spray powders are discussed in U.S. Pat. Nos. 3,617,358 and 4,192,672 and the references cited therein.
For purposes of the present invention, flame spraying may be further subdivided into at least three significant commercial variations according to the nature or velocity of the combustion process, which in turn, affects the coating characteristics.
At one extreme are the simple low velocity processes first developed during the early 1900's, apparently in Switzerland, (see, for example, U.S. Pat. Nos. 1,100,602 and 1,128,058) and still widely used today in various commerical embodiments.
Basically, the low velocity process utilizes a small, often hand-held, device having an open or unconfined flame (such as a modified acetylene torch) to heat and transport a metal powder to a workpiece to form a coating for wear or corrosion resistance. The powder is added to the burning flame near the tip of the torch and thus is heated after leaving the device. Since the coating is usually very porous, another flame is often used to fuse or melt the as-deposited powder into a smoother and more dense coating. This type of process is described in much more detail in U.S. Pat. Nos. 2,526,735, 2,800,419, 4,230,750 and the references cited therein.
At the other extreme, is a complex ultra high velocity process developed by Union Carbide in the 1950's which uses periodic detonation waves moving through a long tube (typically about 1 meter in length) to heat and propel powder from one end of the gun.
The velocity of flame propagation in a detonation is hundreds of times faster than during simple combustion and may be many times the speed of sound. A good discussion of this process may be found in U.S. Pat. Nos. 2,714,563 and 2,774,625.
Intermediate these two extremes, is the more recently developed third type of flame spray process which utilizes high velocities near the speed of sound, produced by continuous combustion, not periodic detonation, in a short tube or duct.
This high velocity process utilizes a more massive water-cooled structure having an enclosed combustion chamber, and optionally, an exit nozzle (like a rocket) to accelerate the oxy-fuel flame, and the powder carried therein, to velocities about five or ten times faster than the unconfined flame of the low velocity process. While the temperature of combustion is thought to be about the same for all types of processes, (about 3000.degree. C.) the high velocity processes seem to increase the apparent temperature of the powder less than the low velocity process; probably because of the shorter time available for heating in the hot gas region. However, the combined high velocity and high temperature produce a much denser high quality deposit on the workpiece.
This improved type of oxy-fuel combustion system is described in more detail in U.S. Pat. Nos. 2,990,653, 4,342,551, 4,343,605, 4,370,538 and 4,416,421.
These three major variations of flame spray coating systems each have certain advantages and disadvantages.
Equipment for the low velocity process is very inexpensive and easy to operate but the coatings produced are usually porous and of low quality. Further, a limited number of materials may be sprayed and the metal deposition rate is low due to the low energy input of the burning gases.
Equipment for the ultra high velocity detonation process is complex, expensive and not usually available for sale but the coatings are of high quality. Further, many different types of materials may be sprayed but again at a low deposition rate.
The intermediate velocity process is also intermediate in cost and complexity. Many types of metallic coating materials may be deposited at high rates and at good densities. However, the very high fuel and oxygen consumption results in a somewhat high hourly operating cost.
Prior to the introduction of plasma-arc spraying equipment, high quality (i.e. dense) coatings which use powder as the sprayed material could only be made utilizing a detonation-gun process.
The plasma-arc spraying process provides coatings of somewhat less quality and has a relatively high equipment cost as well as high hourly operating costs.
Many flame spray applications do not require detonation-gun quality coatings. However, prior to the use of the improved oxy-fuel system operating at above critical or sonic velocity, the available low velocity combustion devices produced coatings of much lower quality than even plasma-arc spraying.
Thus, it is one object of this invention to provide an oxy-fuel combustion system capable of producing good quality coatings at reasonable cost. Another object of this invention is to provide a simple air-cooled device having better thermal efficiency than a water-cooled device.
Some prior work has been done in an effort to improve the flame spraying process but no one has heretofore recognized the source of the problems or the advantages of the present invention.
From the earliest days, it has been known that a blast of compressed air may help shape and/or accelerate the particle stream. See, for example, U.S. Pat. Nos. 2,108,998, 2,125,764 and 2,436,335.
There are also a few devices which use a combustion process to produce a hot blast gas instead of the more common compressed air source which produces a cold blast. See, for example, U.S. Pat. Nos. 4,358,053 and 4,370,538.
Some prior devices also use cold blast gas or the combustion air to cool the gun and/or further heat the particles. See, for example, U.S. Pat. Nos. 2,125,764; 4,187,984 and 4,342,551.
However, none of these prior devices disclose the important relationships between the velocities and temperatures of the heating gas and the blast gas.